The objective of this Phase II SBIR application is to test NTX-1 fiber in patients with constipation-predominant IBS (IBS-C) and determine if it can reduce symptoms, beneficially modify gut microbiota composition, and improve gut milieu in these patients. Our hypothesis for the Phase II study is that NTX-1 fiber is better tolerated (i.e. has less side effects) and is more effective than psyllium (the most commonly used fiber in US) for symptom improvement and is effective in correcting the abnormal colonic milieu in those with dysbiosis and low SCFA (short chain fatty acid) levels. To this end, we propose to conduct an 18-week (2-week initial run-in phase, 12-week treatment phase, and 4-week follow up phase), 2-arm, randomized, double-blind, controlled Phase 2 trial in 120 patients with IBS-C.